Heat a little olive oil over medium heat in a pot.
Once hot fry onion until translucent.
Add 2.5 cups of water and bring to the boil.
Add 1 cup corn meal and stir until water has been completely absorbed.
Reduce heat to low, cover and let corn meal simmer for 15-20 minutes, stirring occasionally.
Season with salt and stir vigorously.
Pour cooked corn meal (polenta) onto a baking sheet and pat flat (as shown in picture above).
Let cool for at least 30 minutes.
Pour a cup of balsamic vinegar into a large saucepan, bring to the boil and then reduce heat to mere simmer and leave uncovered so water can evaporate.
Reduction is ready when it covers the back of a spoon (as shown in picture above).
Peel and crush garlic.
Wash, dry and slice mushrooms.
In a medium saucepan heat a little olive oil over medium heat and fry garlic and mushrooms until soft.
Season with salt and pepper.
Cut polenta into desired shapes and grill in a grilling pan.
Arrange arugula, polenta and mushrooms on a plate and pour balsamic reduction over everything. (If reduction has hardened, heat again and it will become creamy again)